Capacitive pressure sensors are used to measure pressure in many industrial sectors. They frequently feature a ceramic pressure measuring cell as transducer for the processing pressure and evaluation electronics for signal processing.
Capacitive pressure measuring cells consist of basic ceramic elements and a membrane, with a solder glass ring is arranged between the basic element and the membrane. The hollow space between the basic element and the membrane obtained in this way makes possible the longitudinally directed mobility of the membrane as a result of the influence of a pressure. Electrodes, which together form a measuring capacitor, are respectively provided on the underside of the membrane and on the opposite upper side of the basic element. A deformation of the membrane, which has as a consequence a change in the capacity of the measuring capacitor, occurs under the effect of pressure.
The capacity change is registered with the aid of an evaluation unit and converted into a pressure measuring value. These pressure sensors serve as a rule for monitoring or controlling processes. They are therefore frequently connected to higher ranking control units (SPS).
From DE 198 51 506 C1 is known a capacitive pressure sensor, in which the pressure measuring value is determined from the quotient of two capacity values, a measuring capacitor and a reference capacitor. A pressure measuring cell is however not particularly described in this patent publication, but the depicted circuit and the described method are suitable for capacitive pressure measuring cells.
From EP 0 569 573 B1 is known a circuit arrangement for a capacitive pressure sensor, in which a quotient method is likewise used for pressure evaluation.
Quotient methods are based, as a rule, on the following pressure dependencies:
      p    ∼                            C          R                          C          M                    ⁢                          ⁢      or      ⁢                          ⁢      p        ∼                            C          R                          C          M                    -              1        ⁢                                  ⁢        or        ⁢                                  ⁢        p              ∼                            C          M                -                  C          R                                      C          M                +                  C          R                      ,wherein CM identifies the capacity of the measuring capacitor, CR identifies the capacity of the reference capacitor, and p identifies the processing pressure to be determined. The possibility of exchanging CM and CR in the quotient is also conceivable. The disclosed example with CM in the denominator represents, however, the most common form for the benefit of the self-linearization. This embodiment will be assumed in the following, unless otherwise indicated.
The reliability of capacitive sensors is gaining increasingly in importance. One problem of capacitive pressure sensors operating according to the quotient method is that a medium ingress—caused by membrane rupture or made possible by an eventual venting channel—could not be detected as a result of the quotient formation, because the dielectric constant εr correspondingly changes in the numerator as well as the denominator. This problem is further aggravated if the εr of the incoming medium differs only to a minor extent from the εr of air. This is especially the case when the medium to be measured is oil. The εr of oil is typically between 2 and 4, while the εr of air is 1.